Re: [RFC PATCH 4/6] PM / Runtime: Introduce flag can_power_off

[Rafael J. Wysocki]

On Tuesday, February 14, 2012, Zhang Rui wrote:
> Hi, Alan,
> 
> On ä, 2012-02-13 at 15:41 -0500, Alan Stern wrote:
> > On Mon, 13 Feb 2012, Rafael J. Wysocki wrote:
> > 
> > > > I'm not sure if this is really the right approach.  What you're trying 
> > > > to do is implement two different low-power states, basically D3hot and 
> > > > D3cold.  Currently the runtime PM core doesn't support such things; all 
> > > > it knows about is low power and full power.
> > > 
> > > I'd rather say all it knows about is "suspended" and "active", which mean
> > > "the device is not processing I/O" and "the device may be processing I/O",
> > > respectively.  A "suspended" device may or may not be in a low-power state,
> > > but the runtime PM core doesn't care about that.
> > 
> > Yes, okay.  We can say that this patch tries to implement two different 
> > "suspended" states, basically "low power" and "power off" (or D3hot and 
> > D3cold).
> > 
> Right!
> 
> > > > Before doing an ad-hoc implementation, it would be best to step back
> > > > and think about other subsystems.  Other sorts of devices may well have
> > > > multiple low-power states.  What's the best way for this to be
> > > > supported by the PM core?
> > > 
> > > Well, I honestly don't think there's any way they all can be covered at the
> > > same time and that's why we chose to support only "suspended" and "active"
> > > as defined above.  The handling of multiple low-power states must be
> > > implemented outside of the runtime PM core (like in the PCI core, for example).
> > 
> > That's the point.  If this is to be implemented outside of the runtime
> > PM core, should the patch be allowed to add new fields to struct
> > dev_pm_info (which has to be shared among all subsystems)?
> > 
> Surely it shouldn't in this case.
> 
> > Or to put it another way, if we do add new fields to struct dev_pm_info
> > (like can_power_off) in order to help support multiple "suspended"  
> > states, shouldn't these new fields be such that they can be used by
> > many different subsystems rather than being special for the
> > full-power/no-power situation?
> > 
> My opinion is that the concept of "no-power state" is unique for all
> devices/buses/platforms.

No, it is not, basically because of power domains.  If they are used,
then individual device power states are not well defined at all.

> If any of them support this, they can use the routines without any
> confusion.

No, they can't.

Thanks,
Rafael
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